Methionine adenosyltransferase (MAT) is a critical cellular enzyme that catalyzes the formation of S- adenosylmethionine (SAMe). In mammals, two different genes, MAT1A and MAT2A, encode for two homologous MAT catalytic subunits, ?1 (forms either a dimer MATIII, or tetramer MATI) and ?2 (MATII);while a third gene MAT2B, encodes for a regulatory subunit ? that regulates MATII. MAT1A is expressed mostly in liver whereas MAT2A is widely distributed. This grant is currently in its 14th year and has supported 48 original papers and 23 reviews. We described a switch from MAT1A to MAT2A expression in human hepatocellular carcinoma (HCC), which is important as increased MAT2A expression facilitates cancer growth. Patients with chronic liver disease have reduced hepatic SAMe level due to decreased expression and activity of MAT1A- encoded isoenzymes. To better understand the in vivo significance, we created the MAT1A knockout (KO) mouse model. MAT1A KO mice exhibit increased oxidative stress, predisposition to injury and spontaneously develop steatohepatitis and HCC. Over the past funding cycle we have defined several signaling pathways that are abnormal in this model that can contribute to HCC development. To understand how these MAT genes are transcriptionally regulated, we have cloned and characterized the MAT1A, MAT2A and MAT2B promoters. In the past funding cycle we defined how these genes are transcriptionally and post-transcriptionally regulated. While cloning the MAT2B promoter, we uncovered several splicing variants and that the two dominant variants (V1 and V2) are predominantly nuclear and regulate many important cellular processes besides just the activity of MATII. V1 and V2 both regulate cell growth and V1 also regulates apoptosis. We also have novel preliminary data that show 1) MAT genes may be regulated by several miRNAs, some of which have no known function, 2) MAT1A-encoded protein is also present in the nucleus, interacts with several proteins and may regulate growth independent of SAMe, 3) MAT2B interacts with G-protein-coupled receptors kinase-interacting protein 1 and c-Jun N-terminal kinases. Our current application is based on these published work and novel unpublished observations to define the functions and regulations of MAT genes and how their dysregulation leads to liver disease and cancer. Four specific aims are proposed: 1) examine regulation of MAT genes by miRNAs, 2) elucidate the molecular mechanisms of MAT1A-regulated growth and angiogenesis, 3) define regulation of MAT2B and molecular mechanisms of MAT2B variants-mediated growth and apoptosis, and 4) solve the crystal structures of MAT2B V1 and V2. Since this grant began 14 years ago we have uncovered many important and novel functions of these MAT genes and demonstrated their relevance in human liver disease, particularly liver cancer. This application represents ongoing effort to achieve our ultimate goal, translating results from the laboratory to the bedside to prevent complications of liver injury and improve treatment of HCC, topics that are highly relevant to public health. PUBLIC HEALTH RELEVANCE: Methionine adenosyltransferase (MAT) is an essential enzyme as it is responsible for the formation of S-adenosylmethionine, a key molecule that regulates numerous processes including growth and death. The genes that encode for MAT are MAT1A, expressed by normal liver, and MAT2A, expressed in all non-liver tissues, and MAT2B, which encodes for a regulatory subunit that controls the activity of MAT2A-encoded enzyme. Abnormal expression of the MAT genes has important implications in human liver diseases and hepatocellular carcinoma (HCC). Patients with chronic liver diseases have reduced MAT1A expression and in HCC, MAT2A and MAT2B expression are increased while MAT1A is silenced. These changes favor liver cancer cell growth. The goals of this project are to understand how the MAT genes are regulated, and how they control growth, death, and carcinogenic pathways. Successful completion of this project will not only improve our understanding of the role of MAT genes in liver health and disease, but also allow us to design novel therapies against liver diseases and HCC.